As the nervous system develops, synaptic connections between nerve cells are initially established in a highly specific manner. These initial contacts can then be modified to a limited extent during early postnatal life, increasing their specificity still further. Synaptic connections in the spinal cord between muscle sensory afferent fibers and motoneurons, connections that form the neuronal basis of the stretch reflex, provide an excellent experimental system for studying synaptic specificity at the level of single, functionally identified cells. In this project, I am (1) determining the precise pattern of these synaptic connections in adult bullfrogs using both anatomical and physiological techniques; (2) studying the formation and possible rearrangements of these connections during normal development; and (3) assessing the importance of appropriate peripheral targets in determining the final pattern of central synaptic connections.